Typically, designs for index-guided semiconductor laser diodes are based mainly on lowering the laser's threshold current density (for optical computing applications). In order to reduce the volume of active material, the standard approach is to employ a quantum-well active region, resulting in the transparency condition being met for smaller currents. However, as the thickness of the active layer is reduced, the overlap, .GAMMA., of the optical field with the active region also decreases, resulting in an increase in the threshold current for very small active layer thicknesses. To alleviate this problem, single quantum-wells are typically enclosed within a separate-confinement heterostructure (SCH) region so as to increase .GAMMA. to .about.0.03 and .theta..sub.t (the far-field beam divergence in the plane perpendicular to the laser junction) to .about.35.degree..
For optical recording applications a desired feature is the quality of the output beam as typified by the far-field divergence ratio, .rho.=.theta..sub.t /.theta..sub.1, where .theta..sub.1 is the beam divergence in the plane parallel to the laser junction. Since the optimum value for .rho. is 1.0, for large .rho. values (i.e., &gt;3.0), complex optics systems must be utilized between the laser diode and the recording media to correct the shape of the laser's output beam. Consequently, taking .rho. to be 2.8 and assuming the above .theta..sub.t value of 35.degree., .theta..sub.1 must be 12.5.degree.. For the lateral beam divergence to be this size requires either a large lateral index step and/or a small lateral waveguiding thickness (e.g., the thickness of the rib in a ridge waveguide laser diode). By employing a buried heterostructure laser diode scheme both of these criteria can be easily met, however at the expense of ease of fabrication. On the other hand, the ridge waveguide laser diode is relatively simple to manufacture, but .theta. .sub.1 is typically limited to values less than 10.degree..
Recently (M. Yuri, A. Noma, I. Ohta, and M. Kazumura, `Reduction of beam divergence angles perpendicular to the junction planes by modulating the refractive index profile in AlGaAs laser diodes`, presented at the Fall 1991 meeting of the Japanese Society of Applied Physics), a buried ridge waveguide laser diode was designed with a small transverse divergence (.theta..sub.t was .about.14.degree.), thus, easing the restriction on the magnitude of .theta..sub.1. Their solution was to grow depressed-index cladding layers on both sides of the active region, as shown schematically in FIG. 1 for an AlGaAs-based laser diode. In the figure is indicated the relative Al content of the various layers, where 10 refers to the n+-GaAs substrate. On the surface of 10 is formed the lower cladding layer 12. Upon 12 is deposited the lower depressed-index cladding layer 14. The index of refraction of this layer is smaller than that of the surrounding layers since the index of refraction of AlGaAs materials is smallest for pure AlAs. On the surface of 14 is tonned the lower spacer layer 16, followed by the active region 18 and the upper spacer layer 20. Upon 20 is tonned the upper depressed-index cladding layer 22 followed by the upper cladding layer 24. Lastly, upon the surface of 24 is formed the capping layer 26. Since light tends to avoid low-index regions, the physical effect of the inclusion of the depressed-index cladding layers is to push the transverse-confined waveguide mode both toward the middle and ends of the structure. With greater light intensity present in the upper and lower cladding layers, .theta..sub.t decreases as desired. .GAMMA. remains approximately stationary since light is also pushed towards the middle (active layer) of the structure. More specifically, it was recently (T. Cockerill, J. Honig, T. DeTemple, and J. Coleman, `Depressed index cladding graded barrier separate confinement single quantum well heterostructure laser,` Appl. Phys. Lett., vol. 59, 2694, 1991) determined that for a broad-area graded index separate-confinement heterostructure (GRINSCH) device, .theta..sub.t was 27.degree. and 59.degree. for structures with and without the inclusion of the depressed-index cladding layers, respectively.
The device structure of Yuri et al. has the desirable trait of a small universe beam divergence; however, the buried ridge structure is difficult to manufacture. Thus, it would be advantageous to employ the depressed-index cladding layers in a ridge waveguide laser diode. Unfortunately, incorporating the layers into a ridge waveguide design is not straightforward. One difficulty is that in order to get sufficient interaction of the modal-field with the rib structure,, the rib etching must extend deep within the upper cladding layer, i.e., terminate just short of the upper depressed-index cladding layer. Hence, there must be tight control on the etching process: underetching will produce weak lateral confinement, while overetching into the upper depressed-index cladding layer will result in destabilizing the lasing mode. Another problem results from the modal-field having enhanced tails as discussed above. In order to avoid absorption losses in the capping layer, it is necessary to increase the thickness of the upper cladding layer to .about.2.4 .mu.m for the structure indicated in FIG. 1, which, together with the deep rib etch, results in rib heights on the order of 2.5 .mu.m. Using conventional wet chemical etching techniques, large rib heights are difficult to produce, especially if one desires rib widths on the order of 3.0 .mu.m.